The invention relates to a circuit configuration for detecting the state of at least one electrical actuating element, in particular for detecting an actuation of switching elements in a motor vehicle. Such circuit configurations generally have a signal input for receiving an input signal representing the state of the actuating element, a signal output for emitting an output signal representing the state of the actuating element, and a control output for emitting an activation signal for an evaluation unit, in order to change over the evaluation unit from an inactive operating state to an active operating state. A control unit is provided, which is connected to the signal input on the input side and to the control output on the output side and serves for generating the activation signal in a manner dependent on the input signal. The input signal present at the signal input is an analog signal, the control unit generates the activation signal for the evaluation unit if the input signal lies within a predetermined range of values.
In modern motor vehicles, electrical loads, such as, for example, the interior lighting, are controlled by a microprocessor in a manner dependent on the switching state of a plurality of switching elements such as, for example, door contact switches, trunk closure switches and ignition lock switches. This has the disadvantage that the microprocessor consumes current in the course of checking the switching state of the individual switching elements, which leads to considerable loading on the vehicle battery in particular when the motor vehicle is not used for a relatively long time.
In order to solve this problem, German Patent DE 44 14 734 C2 discloses a circuit configuration which monitors the switching elements and switches the microprocessor into an active operating state only in the event of an actuation of one of the switching elements, whereas the microprocessor is otherwise operated in a so-called standby mode with a low current consumption. This significantly reduces the current consumption of the microprocessor during the times when the motor vehicle is not used. In this case, the switching state of the individual switching elements is monitored by clocked voltage pulses being applied to the switching elements in each case via series resistors, a voltage measuring unit measuring the voltage that is dropped across the individual switching elements and represents the switching state of the respective switching element. Thus, the electrical potential at the input of the voltage measuring unit is pulled to ground when one of the switching elements is switched on, whereas the electrical potential at the input of the voltage measuring unit corresponds to the supply voltage when the switching elements are open. The known circuit configuration thus enables a current-saving interrogation of digital switching elements with two switching states.
What is disadvantageous about the known circuit configuration it described above is the fact that, at the signal output, an output signal representing the state of the switching elements is generated even when no switching element at all is actuated and, accordingly, there is also no need for an output signal that represents the state of the switching elements. The known circuit configuration is thus always in an active operating state during the clocked voltage pulses, which results in that electrical energy is consumed unnecessarily.
Published, Non-Prosecuted German Patent Application DE33 12 153 A1 discloses a circuit configuration for the interrogation of a plurality of pushbutton switches, which has an evaluation unit, and the evaluation unit can be changed over from an inactive current-saving operating state to an active operating state. For this purpose, the known circuit configuration has a control unit that switches on the operating voltage for the evaluation unit when one of the pushbutton switches is actuated, with the result that the evaluation unit can evaluate the switching states of the individual pushbutton switches. This has the disadvantage that before the evaluation unit is activated, the entire voltage supply must be run up, with the result that the evaluation unit can be activated only with a specific time delay.
Finally, Published, Non-Prosecuted German Patent Application DE 199 17 819 A1 which was published after the priority date of the instant application, discloses a circuit configuration for detecting switching states of switches for the activation of motor vehicle electronics, the circuit configuration generating a wake-up signal if at least one switch undergoes a transition from a first defined switching state to a second defined switching state. The circuit configuration is configured such that it generates the wake-up signal even in the event of a transition from the second switching state to the first switching state.
It is accordingly an object of the invention to provide a circuit configuration for detecting the state of at least one electrical actuating element which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which the electrical energy consumption is minimal and the evaluation unit can nevertheless be activated rapidly.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit for detecting a state of at least one electrical actuating element. The circuit contains a signal input connected to the actuating element for receiving an input signal representing the state of the actuating element, the input signal being an analog signal. A signal output for emitting an output signal representing the state of the actuating element, is provided, along with an evaluation unit. A control output is provided for emitting an activation signal for switching the evaluation unit from an inactive operating state to an active operating state, and the control output is connected to the evaluation unit. Control circuitry is provided and has an input side connected to the signal input and an output side connected to the control output. The control circuitry generates the activation signal in a manner dependent on the input signal. The control circuitry generates the activation signal for the evaluation unit if the input signal lies within a predetermined range of values. A controllable switching element is connected between the signal input and the signal output and, depending on a switching state of the controllable switching element, the controllable switching element enables an active operating state or an inactive operating state of the circuit. The output signal representing the state of the actuating element is emitted only in an active operating state of the controllable switching element.
The invention contains the general technical teaching of not providing a discriminator with a predetermined threshold value for the interrogation of the actuating elements, but rather of providing a circuit that is insensitive to component fluctuations of the forward and/or reverse resistance of the it actuating elements.
Preferably, a plurality of switching elements each having different reverse and forward resistances are connected in parallel at the signal input of the circuit configuration as according to the invention.
In the preferred embodiment of the invention, the signal input of the circuit configuration according to the invention is connected to the signal output of the circuit configuration via a voltage divider, in order likewise to be able to output an analog signal on the output side.
According to the invention, the controllable switching element is in this case disposed between the signal input and the signal output, and, depending on its switching state, optionally enables an active operating state or an inactive operating state of the circuit configuration. The-output signal represents the state of the actuating element being emitted only in the active operating state of the switching element. Thus, the circuit configuration according to the invention permanently detects, on the input side, the input signal representing the state of the actuating element, in order to be able to change over a separate evaluation unit upon activation of the actuating element to an active operating state. The evaluation unit contains, for example, a microprocessor and an associated power supply, the evaluation unit, upon activation, also activating the circuit configuration according to the invention, in that the control input of the switching element disposed between the signal input and the signal output is correspondingly driven.
The controllable switching element of the circuit configuration according to the invention is preferably a transistor, which may be embodied as an N-channel MOSFET, for example.
In the preferred embodiment of the invention, the contact resistance of the controllable switching element of the circuit configuration in the activated operating state is 1xcexa9, in order that the output signal is corrupted as little as possible. In this case, currents of up to 20 mA flow via the N-MOSFET, which generates a still acceptable voltage drop of up to 20 mV. A larger contact resistance of the N-MOSFET would cause an additional error in the voltage measurement.
In accordance with an added feature of the invention, a voltage terminal for receiving a voltage source is provided. A resistor is connected between the signal input and the voltage terminal, in order that the signal input, during an inactive operating state of the controllable switching element, is kept essentially at a predetermined voltage level of the voltage source.
In accordance with an additional feature of the invention, the control circuitry has a series circuit containing a resistor, a capacitor and a diode connected to the signal input, and the resistor is connected to the voltage terminal. The series circuit has a voltage tap connected to the control output.
In accordance with a further feature of the invention, a ground terminal is provided and a buffer capacitor is connected between the signal input and the ground terminal. Additionally, a further buffer capacitor is connected between the signal output and the ground terminal.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for detecting the state of at least one electrical actuating element, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.